That isn't an easy question to answer. Personally, customs are better, but that is because my ear canals are tiny. Most universals will after a time, cause irritation. That said, the CT6 cable is no where near as good. And, the CK100, while a bit more 'non linear', is more dynamic than the CT6. The CT6 is like a less dynamic JH13Pro.
It is a hard decision between the two. If Sleek had a better cable, I would probably side with the Sleek since its sound is completely in line with traditional earphones. But, the CK100 represents a unique value.
I know that ponderous doesn't think so, but I don't think he/she takes into account that the CK100 isn't competing with UM3x etc. on their shores. In Japan (the only country which officially sells the CK100), it sells for just over 300$. It is cheaper street than any of its competition and better made. Sound is up to you of course, but I think it is better to leave that moot for the moment.
It is unfair to say it is expensive when the only places you can grab it are importers. They set the price, not Audio Technica. Not consumers. Not the earphone. The earphone is sold on average for less than 400$. It would be just as fair to say that the UM3x which here sells for more than 500$ is not worth it because it is so expensive. In fact, it would be fair in the same way to say that all earphones should only be valued by their most expensive importer, not by the street price or the MSRP. If so, the Shure SE530 would still be 449$ or more and many places would sell other top tier universals for more than 500$. Everything would be neutralised again.
The fact is that the CK100 is only officially for sale in Japan. Exporters drive whatever price they want at their own detriment. If they sold closer to retail or street price, they would sell more and the CK100's name wouldn't be tarnished by grubby exporters.